(1) Field of the Invention
The present invention relates to a boiler and more particularly to a double-tube type waste heat boiler.
(2) Description of the Prior Art
Heretofore, from the standpoint of savings and recycling use of thermal energies, waste heat boilers have been put in use. With increasing interest in preservation and saving provoked by natural resources of late, there has been added the significance of waste heat boilers to cope with such circumstances and accordingly, it is so expected that the demands for large-sized, high-temperature and high-pressure designs of such waste heat boilers will become even greater hereafter. There are known general technical difficulties in the designs and constructions of waste heat boilers, among which the most common yet significant problems are such that tubings and tube sheets of the boilers subjected to hot gases are heated to high temperatures, thus inevitably resulting in the loss of strength thereof, and that stresses are produced due to differential thermal expansion from uneven temperature distribution in the metal parts organizing the boiler, i.e., a boiler drum, boiler tubes and tube sheets. In the attempt to meet such problems, as the boilers are recently designed to be larger and for use at high temperatures, reconsideration as to boiler design and construction, selection of suitable materials, increase in the material weight, troublesome and complex installation and inspection, etc., which would undoubtedly lead to increased costs and expenses in the manufacture, inspection and maintenance thereof must be considered. There are a variety of designs and constructions of waste heat boilers, among which the most typical one is such that there is provided a steel cylindrical shell or drum, having two tube sheets or plates with a plurality of openings welded or otherwise connected in an opposed relation with both ends of the boiler, and having a plurality of tubes within the boiler drum securely connected at each opening of the tube plate, respectively, wherein water is supplied from one end to the other of the boiler drum and high temperature waste gases are fed externally from one end tube plate through the plurality of tubes within the drum so as to provide heat to water within the drum and thus generate water vapor to be taken out from the other end of the drum and thereafter, the waste gases are directed outwardly from the opposite end tube plate to be discharged to a following processing step.
Now, referring to FIG. 3, a fragmentary cross-sectional view shows the conventional waste heat boiler in use and same is typically constructed in such a manner that there is a tube plate 21 connected at end of a boiler drum 20, the tube plate 21 having a plurality of tubes 22 rigidly connected thereto. There is provided a vacancy or space 24 defined by a channel 14 lined with an insulating or refractory material 15 on the exterior surface of the tube plate 21 as shown, into which high temperature waste gases are introduced and through the plurality of tubes 22 so as to be heated by transfer water within the drum 20, thereby generating water vapor. In such a construction, since the tubes 22 are subjected to high temperature gases while the boiler drum 20 is subjected to cold water, there is naturally a substantial difference in amounts of expansion between the drum 20 and the tubes 22 and as a result of such differential expansions, there occur substantial stresses at and near the connections between the plate 21 and the tubes 22 and the junction 23 between the plate 21 and the drum 20. It is noted that when there is a great difference in temperatures of the waste gases and water, or when the drum 20 or the tubes 21, having a substantial thickness, are under high water pressure, or when a waste heat boiler is of a large size, large amount of stress increases to such an extent that same eventually becomes an obstacle to the design thereof. In the case where it is of a high-pressure design, there is further stress from the increased inner pressure in addition to the stresses caused by the thermal expansion mentioned above and thus particularly resulting in further total stress at and near the junction part 23. Consequently, there is induced complexity in the configurational and structural designs of and around the junction part 23, which further makes it necessary to severely analyze possible fatigues of the material therearound, thus bringing technical difficulties in the selection of material to be used as well as in such field operations as manufacturing inspection, repairing, and so on, thus resulting in eventual increased costs and expenses of manufacture and maintenance of the boilers. Also, the tube plate 21 is designed to face high temperature gases through the refractory material 15, but still heated to a substantially high temperature on the face thereof and therefore, it would be likely lose its strength. Consequently, it is generally necessary to design the tube plate to be substantially thick, however, such a thick tube plate would necessarily bring reduced efficiency in cooling which would be effected by water behind this thick plate, thereby possibly causing the temperature of the tube plate to be further higher. For the practical design of a boiler, it is therefore essential to resolve all such technical problems.